D. External links

[0] Creative Commons BY SA license:
https://creativecommons.org/licenses/by-sa/3.0/

[1] HTTP basic access authentication:
https://en.wikipedia.org/wiki/Basic_access_authentication/

[2] OpenTravel Alliance:
https://opentravel.org/

[6] browsable interface to the OTA XML schema files (choose model "OTA2015A"):
https://www.pilotfishtechnology.com/modelviewers/OTA/